It is very important in connection with internal combustion engines to be able to determine when the filter in the air cleaner needs replacing. In connection with automotive vehicles, this is often done in a gasoline service station by removing the cover and holding a light on one side of the filter element to see the extent to which the light is visible on the other side. This method is not only inaccurate, but also involves removing the cover to the air cleaner. Consequently, this check is often neglected. There is very little question but what a dirty filter in an air cleaner of an internal combustion engine greatly increases the amount of pollutants which are discharged into the atmosphere and decreases the efficiency of the engine.
Various arrangements have been proposed for checking the condition of automobile filters. One such arrangement, shown in the McKinlay U.S. Pat. No. 3,317,187 and U.S. Pat. No. 3,381,651, is that of providing a differential air pressure responsive device built into the cover of an air cleaner. This, however, involves either a very substantial modification of the air cleaner cover or the provision of a special cover.
Another arrangement which has been proposed is that shown in the Anderson et al U.S. Pat. No. 4,020,783. In this case, a wingnut incorporating a differential pressure responsive device is employed to hold in place and detect whether an inner safety filter is dirty or not. To determine, however, whether this inner filter is dirty or not requires that three covers be removed. The suggestion is made in a later portion of the specification that the special wingnut can also be employed to replace the wingnut holding the outer filter in position. There is still no concept, however, of having an indicator visible from outside the air cleaner. Furthermore, the special nut of the Anderson et al patent would not be adaptable to hold the outer cover of a conventional air cleaner in position.